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Rare green comet makes closest approach to the sun

The comet C/2022 E3 (ZTF) will be at its closest to the sun since the
last ice age before it swings past Earth in February.







This may very well be the last time that C/2022 E3 comes our way again.







Typically, during the course of a year about a dozen comets will come within the range of
amateur telescopes. Most quietly come and go with little fanfare,
but some are particularly noteworthy.

During the upcoming weeks, a newly-discovered comet will be making a relatively close
approach to the Earth. On Feb. 1, comet C/2022 E3 (ZTF) will pass to within 28 million miles
(42 million km) of our planet, its first approach in 50,000 years. While this will no doubt
entice many skywatchers to attempt to view the comet, whether or not one will actually
be able to see it will depend on a variety of factors including location and light pollution
from both natural and artificial sources.

But don't be dismayed! Even if you don't have the right gear or conditions to see comet
C/2022 E3 (ZTF), the Virtual Telescope Project will be hosting a free livestream of the
comet beginning at 11:00 p.m. EST on Jan. 12 (0400 GMT on Jan. 13). You can watch
the live webcast courtesy of the project's website(opens in new tab)
or on its YouTube channel(opens in new tab).


Discovery and history


On March 2, 2022 astronomers Frank Masci and Bryce Bolin using the Zwicky Transient Facility
(ZTF) at the Palomar Observatory in Southern California, came across an object which they
initially identified as an asteroid. It appeared very dim — it was estimated at magnitude
+17.3 — or nearly 25,000 times fainter than stars at the threshold of detectability using just
the human eye. Subsequent observations revealed that this star-like object possessed a very
tightly condensed coma, indicating that it was in fact, a comet. It was the third such object
discovered in the fifth half-month (A, B, C, D, E) of the year, so it received the designation
C/2022 E3 (ZTF). At the time, the comet was situated 399 million miles (643 million km)
from the sun, or just inside the orbit of the planet Jupiter.

After enough observations were gathered to compute an orbit, astronomers determined
C/2022 E3 to have an orbital period of roughly 50,000 years. Its last passage through the
inner solar system apparently came during the Upper Paleolithic or Old Stone Age. If we
take these calculations at face value, then the last people to look up and witness this visitor
from the depths of the outer solar system, were likely very early Homo sapiens or Neanderthals.

But this may very well be the last time that C/2022 E3 comes our way again. The latest
orbital elements suggest that the comet is currently traveling on an orbital path with an
eccentricity of 1.00027, or in other words, a parabolic orbit. Such an orbit is not closed,
so after it sweeps around the sun C/2022 E3 will move back out into deep space, never
to return again. So, this will be the comet's last time to "perform" for us. We know that
comets are composed primarily of frozen gases that are heated as they approach the sun
and made to glow by the sun's light.

We call this cloud of gas the head or coma.


As the gases warm and expand, particles of dust that were embedded in the comet's nucleus
are also released into space. The solar wind blows this material out into an appendage we call
the tail. To observers of antiquity, comets resembled a stellar head trailed by long hair, so they
called comets, "hairy stars."




Comet C/2022 E3 ZTF photographed from Rome, Italy on Sept. 12, 2022.
(Image credit: Gianluca Masi)



Bright among "common" comets


Comets can be broken down into two basic categories:

Bright comets — the kind that can excite those of us without binoculars
or telescopes — appear on average perhaps two or three times every 15 to 20 years.
The last such comet to do that was comet NEOWISE (C/2020 F3) in July 2020.

Then there are the common comets, of which most are only visible either
with good binoculars or a telescope. The vast majority of comets fall into this
category, but C/2022 E3 (ZTF) may end up ranking as exceptionally bright so far
as most common comets go, since for a short while it may hover right at the
cusp of naked-eye visibility (for those fortunate enough to be blessed with dark,
non-light polluted night skies).

For a comet to become readily visible without optical aid, it usually needs to approach
closer to the sun than the Earth (92.95 million miles or 149.56 million km). But at
perihelion (its closest approach to the sun) on January 12th, C/2022 E3 will get no
closer than 103.4 million miles (166.4 million km). It will then begin to move away from
the sun. Most comets, however, continue to remain quite active for a few weeks after
passing the sun and this will be good so far as the comet's visibility for us is concerned.

In fact, during the few weeks following perihelion, the orbital geometry between the
comet and the Earth has the distance between the two rapidly shrink. That distance will
decrease by nearly 40 million miles (64 million km) between Jan. 12 and Feb. 1. As a
result, the anticipated increase in the comet's brightness during that timeframe is
expected to correspondingly increase, perhaps more than five-fold.

Closest approach to Earth (perigee) will come at 1:11 p.m. EST on Feb. 1 at a distance
of 28,390,710 miles (42,471,730 km).




Comet C/2022 E3 ZTF seen on Dec. 22, 2022.
(Image credit: Edu INAF/Wikimedia Commons)



Where to find it and viewing prospects


Right now, C/2022 E3 is a predawn object, located in the constellation of Corona Borealis
at a declination near +34°; it rises in the northeast shortly after midnight. On Jan. 12, the
date of its closest approach to the sun, the comet will have shifted several degrees to the
northwest. From then onward, its movement against the background stars will progressively
increase westward as it approaches the Earth.

The comet will move into northern Boötes on the 14th, and for most mid-northern latitude
locations, it will become circumpolar (remaining above the horizon at all times) by the 20th.

On the nights of Jan. 26 and Jan. 27, it can be conveniently found passing several degrees
to the east of the bowl of the Little Dipper. On the evening of Jan. 27, it will be 3.5° to the
upper right of orange Kochab, the brightest of the two outer stars in the bowl. On the
evening of Feb. 1, when C/2022 E3 (ZTF) is passing closest to Earth, it will be within the
boundaries of the vague and dim constellation of Camelopardalis. By Feb. 5, it will pass
within a couple of degrees to the west of the brilliant yellow-white star Capella and the
next night it will be within the triangle that makes up "The Kids" asterism in Auriga and
will be located almost directly overhead at around 8 p.m. local standard time.

A number of different predictions have been made regarding the brightness of C/2022 E3 (ZTF)
as it passes closest to Earth at the start of February. I believe, based on observations of
C/2022 E3 through early January, that the forecasts of Japanese comet expert Seiichi Yoshida
and Dutch comet expert Gideon Van Buitenen, will be close to the truth, indicating a magnitude
of no fainter than +7.5 during early January and peaking near +5 by the Feb. 1 perigee.

Assuming that its brightening trend remains on target, the comet should become faintly visible
with the unaided eye by the third week of January.



A NASA image showing the path of comet C/2022 E3 ZTF across the January sky
for the Northern Hemisphere.
(Image credit: NASA/JPL-Caltech)



Keep your expectations low


But as compelling as this all may sound; I now must temper any excitement by providing
a very important disclaimer.

Later this month, many people with binoculars and small telescopes will no doubt attempt
to follow the path of C/2022 E3 across the night sky. But actually seeing it will strongly
depend on your observing site. From locations that are plagued by light pollution, I'll bet
that sighting this comet is going to prove to be a rather difficult task. And even for those
who are blessed with dark and starry skies, finding the comet could prove to be a bit of a
challenge. This is because as the comet gets closer to Earth it will become rather large in
angular size — perhaps appearing nearly as large as the moon by the start of February — as
well as appearing rather diffuse.

Indeed, many with little observing experience will sharply question the predictions for a
fifth or sixth magnitude object. But remember, you're not looking for a sharp star-like object,
but rather something which is spreading its light out over a comparatively large area.

In fact, under a completely dark sky, free of light pollution, perhaps the best instruments
for locating the comet will be your own two eyes, especially if you use averted vision.


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Celestial deception!


Recent photographs have shown the comet displaying a distinct greenish color and sprouting
two tails, one of which appears impressively long. Sadly, such long-exposure images tend to
be quite deceptive. For one thing they bring out colors that are not readily evident to the eye.
As to why the comet's head appears green is likely due to a molecule made from two carbon
atoms bonded together, called dicarbon. This unusual chemical process is confined chiefly
around the comet's head, not its tail.

Comets generally throw off two types of tails; tails composed primarily of gas, and tails
composed primarily of dust. Dust tails are far brighter and more spectacular to the eye
than gas tails, because dust is a very effective reflector of sunlight. The most spectacular
comets are dusty and can produce long, bright tails making them awesome and impressive
celestial spectacles.

Gas tails on the other hand appear much fainter and glow with a bluish hue. The gas is
activated by the ultraviolet rays of the sun, making the tail glow in much the same way
that black light causes phosphorescent paint to light up.

Unfortunately, gas tails produced by most comets, appear long, stringy thin, and quite faint;
impressive in photographs but underwhelming visually. And that's what we're currently seeing
with C/2022 E3. The comet is also shedding a brighter dust tail, but at this moment it's
rather short and stubby.

So, most who ultimately locate C/2022 E3 in their binoculars or telescopes will, I believe,
typically describe it as a nearly circular cloud, appearing noticeably brighter and more
condensed near the center. Some might also detect its dust tail appearing as a bit of an
elongation of the comet's coma, but hardly the kind of tail or appendage exhibited by
other larger and brighter comets.



That darn moon!


There is one other factor that will affect whatever views you might get of the comet and
that will be the moon.

From now until about Jan. 15, its bright light will hinder your views of the comet in the
early morning sky, although thereafter it will slim down to a waning crescent and become
progressively less of a hindrance. It will arrive at new phase on Jan. 21. A few days later
it will reappear in the western evening sky as just a thin crescent, but by Jan. 28, it will
again be lighting up the sky during the first part of the night and seriously interfering with
observations of the comet — and just when it is attaining its peak brightness. The moon
will set later in the night, leaving the sky dark during the predawn hours, but as it
approaches full phase on Feb. 5, the amount of time between moonset and the first light
of dawn will get noticeably shorter.

After full moon, dark sky opportunities open up in the evening sky. From mid-northern
latitudes on Feb. 7, there will be about a half-hour window of darkness between the end
of evening twilight and moonrise. Three nights later, C/2022 E3 will be visible from the
end of evening twilight until about 11 p.m. without any lunar interference. At nightfall it
will sit less than 2° to the upper left of bright Mars and will stand 75° above the
south-southeast horizon, and probably hover at around +6.5 magnitude.

If you want to take a look at C/2022 E3 ZTF and don't have everything you need, be sure
to peruse our guides for the best binoculars and the best telescopes to view the comet
or anything else in the sky. For capturing the best comet images you can, we have
recommendations for the best cameras for astrophotography and best lenses for
astrophotography. 



Happy comet hunting!


Editor's Note:

If you photograph comet C/2022 E3 (ZTF), and would like to share
it with Space.com's readers, send your photo(s), comments,
and your name and location to spacephotos@space.com.